It is known that exhaust gas aftertreatment systems of a Diesel engine can be provided with a Lean NOx Trap or LNT. An LNT is provided for trapping nitrogen oxides (NOx) contained in the exhaust gas and is located in the exhaust line. Moreover, such exhaust gas aftertreatment systems are normally provided with a diesel particulate filter or DPF for control of particulates.
An LNT is a catalytic device containing catalysts, such as rhodium, platinum and palladium, and absorbents, such as barium-based elements, which provide active sites suitable for binding the nitrogen oxides (NOx) contained in the exhaust gas, in order to trap them within the device itself. Lean NOx traps are subjected to periodic regeneration processes, whereby such regeneration processes are generally provided to release and reduce the trapped nitrogen oxides (NOx) from the LNT.
The LNT is operated cyclically, for example by switching the engine from lean-burn operation to an operation whereby an excess amount of fuel is available, also referred to as a rich operation or a regeneration phase (DeNOx). During normal operation of the engine, NOx is stored on a catalytic surface. When the engine is switched to rich operation, NOx stored on the absorbent site react with the reductants in the exhaust gas and are desorbed and converted to nitrogen and ammonia, thereby regenerating the absorbent site of the catalyst.
Due to the presence of sulphur into the fuel, an LNT is exposed to the sulphur poisoning, which reduces the NOx storage capacity of the catalyst. In order to re-enable the capability of the aftertreatment system, a desulphation (also called DeSOx regeneration or simply DeSOx) is needed. The DeSOx includes the provision of a fast alternation of lean and rich atmosphere (hereafter, wobbling) at high temperature during engine working conditions.
A generic layout of an aftertreatment system for an internal combustion engine equipped with an LNT and a DPF in closed coupled position (which means the aftertreatment devices are located close to the turbine outlet, at the beginning of the exhaust pipe) includes one canning with lean NOx trap and a particulate filter, one temperature sensor at LNT inlet, one temperature sensor at LNT outlet, one air/fuel ratio sensor at LNT inlet, one air/fuel ratio sensor at LNT outlet and one pressure sensor at LNT outlet. The LNT specific modes, generically called regenerations (namely DeNOx, DeSOx Lean and DeSOx Rich), are obtained through engine actuators (injectors, exhaust gas recirculation valve, variable geometry turbine and others) which are moved to dedicated set points in order to achieve the desired combustion properties, using information mainly coming from the mentioned sensors.
The lean NOx trap is a cost efficient alternative to a selective catalytic reduction (SCR) system, which operates thanks to a fluid catalyst such as urea, or ammonia, or a combination thereof (generally in a water solution) injected into the exhaust line of the engine in order to promote the reduction of NOX. On the other hand, the LNT efficiency and diagnosis is really difficult to be controlled, especially in view of more and more stringent emission limits as foreseen by the Euro 6 emission standard in its second stage, where real driving cycles will be used for the verification of “In Use Compliance” of the emission limits.
Therefore a need exists for a new aftertreatment system and a method of operating the lean NOx trap, which overcomes the above inconvenience, thus avoiding a more expensive system, as the selective catalytic reduction system.